Electrolytic oxidation of water-soluble thioethers



Aug. 10, 1965 J. A. PURsLEY ELEGTROLYTIC OXIDATION OF WATER-SOLUBLE THIOETHERS Filed April 13, 1962 www I.. w

INVENTOR (//f PMM/y ATTORNEYS United States Patent O WATER- This invention relates to an electrolytic process for the oxidation of water-soluble organic sulfides. More particularly it relates to a process for the preparation of organic sulfoxides and sulfones by the anodic oxidation of an aqueous solution of an organic sulfide.

Heretofore sulfoxides and sulfones have been prepared by the chemical oxidation of a thioether. Thus, upon treating a thioether with nitric .acid a sulfoxide is formed. Similarly, controlled oxidation of a thioether with hydrogen peroxide in a solution of either acetic acid or acetone may be used to produce a sulfoxide. By the use of stronger oxidation conditions a sulfone may be produced in like manner. Thus, .substituting concentrated or fuming nitric acid for the regular nitric acid, the product of :oxidation of a thioether is normally the sulfone. Other strong -oxidizing agents such as permanganate, hypochlorite, etc., may be used to produce sulfones. It is understood that regula-r commerc-ial practice in producing organic sultones is to oxidize -a thioether utilizing hydrogen peroxide with an appropriate catalyst.

It has long been known that it is possible by similar rneans Ito produce sulfoxides and sulfones by the controlled electrolytic oxidation of a suitable thioether. Thus Fichter and Wenk in Chemische Berichte 45, 1376-1383 (1912) show the electrolytic oxidation of diethyl sulfide to produce first the sulfoxide and then the sulfone. In this process the diethyl sulfide is dissolved in acetic acid with hydrochloric acid added as the electrolyte and the oxidation carried out with a platinum anode to produce the sulfoxide. vThe sulfoxide is then isolated and placed in water with hydrochloric acid electrolyte and the oxidation completed to the sulfone. Thus the initial electrolytic oxidation is the electrolytic analogue of the earlier controlled oxidation of the sulfide to the sulfoxide using hydrogen peroxide in acetic acid under controlled conditions. The necessity according to this process for carrying out the overall production of the sulone in two steps with an intervening separation of the sulfoxide is highly undesirable and not economical. Accordingly, the electrolytic process for the production of sulfones has neve-r been successful.

it has now been discovered that water-soluble organic sulfides may be oxidized direct-ly to the sulfone by elecftrolytic oxidation. Suitable water-soluble organic sulfides include 2,2dihydroxyethylsulfide, alpha-thiophenic acid, 2-thienylacetic acid, thiodiacetic acid, thiodipropionic acid, ethylthioethanol, the water-soluble sulfides formed by reacting hydrogen sulfide with either two or more mols of an alkylene oxide as propyleneoxide, or with epichlorohydrin, :or with a lactone, etc. According to the process of the instant invention a solution is prepared of the watersoluble organic sulfide, water and a suitable electrolyte and an electric current passed through the solution whereby the sulfide is oxidized to the sulfoxide or vthe sulfone at the anode. Water-solubility is generally imparted to organic compounds by the presence of hydrophilic substituents in the compound. Such substituents are often themselves oxidizable. However, the instant process enables the thioether group to `be smoothly oxidized to the sulfone with exceptionally high efiiciency and a minimum of by-products Without interference from such substituents.

The function of the electrolyte in the instant process is believed to be merely that of carrying current so that it is not necessary that the electrolyte form a .per comaantast Patented Aug'. l0, 1965 "ice pound. However, the precise nature of the reaction is not now known and applicant does not Wish to be limited t0 any particular theory of operation. Suitable electrolytes include water-soluble compounds of the following anions: chloride, hypochlorous, chlorate, perchlorate, fluoride, sulfate, persultate, etc. any anion may be used which has 21 discharge potential greater than the oxidation potential of the organic sulfide being oxidized, but less than the oxygen overpotential on the particular anode in use. Peroxygenated intermediate anions such as persuliate are not essential as for example in the manufacture of hydrogen peroxide. The cation must form a water-soluble compound with the particular anion and desirably is selected to have a low discharge potential. The ionized salt is the electrolytic conductor and its concentration is not critical. It may vary from about a fraction of 1% up to a saturated solution. Generally the presence of the organic sulfide dissolved in the water limits the solubility of the electrolyte. As is evident, the lower the concentration of electrolyte, the less the current-carrying capacity of the solution, and hence the slower the electrolytic process; the larger the concentration of electrolyte, the faste-r the electrolytic reaction. Accordingly, the maximum concentration of electrolyte used is generally dictated solely by commercial convenience. Generally, the concentration will vary from rabout 0.1% by Weight ofy the total solution up to a saturated solution of the salt.

The concentration of the water-soluble organic sulfide similarly may Vary over Wide limits, .i.e., from about 1% by weight up to the limit of solubility of the sulfide with the aqueous electrolyte solution. The minimum concentration of sulfide used will generally be limited by the depolarizing ability of the sulfide. Usually, high concentrations of organic sulfide result in decreased conductance, i.e., current does not pass as readily through high concentrated sulfide solutions, so that for mos-t efficient power use lower concentrations are sometimes desirable. Thus the maximum concentration of organic sulfide in water is imposed either by its solubility characteristics, or, in cases where the .sulfide is very soluble in water, by commercial considerati-on of efiicient power use. Ordinarily, I employ solutions containing from about 30 to about 80% by weight of organic sulfide. Lower concentrations may :also be desirable where the sulfide might unduly limit the solubility of the electrolyte.

Electrolytic oxidation, in accordance with this invention, is ordinarily effected at about room temperature, but this is not critical and the temperature can vary over a considerable range, as from about 5 C. to about 150 C. The precise limit will vary according to the nature of the materials being oxidized. Thus the lower temperature limit is imposed by the freezing point of the solution, while the maximum operating temperature is imposed by the thermal stability of the material being oxidized. The use of higher temperatures may require the use of pressurized containers to prevent escape of vapors, which in turn requires larger equipment cost. The passage of current during the electrolytic oxidation necessarily will heat the solution so that under normal operation conditions the solution will normally be above room temperature. Most of the organic sulfides contemplated herein have sufiicient thermal stability so that the electrolytic oxidation may proceed at the elevated temperatures encountered in such reactions without the use of extensive cooling although some cooling is desired for temperature control and lower temperatures are usually preferred. Desirable temperatures fall within the range of about 25 C to about C.

Suitable anode materials which have been used include platinum7 lead and carbon. In general the anode should be so selected as not to be attacked during the reaction.

' The nature of the cathode is not critical.

Materials such as nickel have proven eminently satisfactory without the necessity for more expensive platinum or gold. i

In carrying out the process of the instant invention, the Water-soluble organic sulfide is iirst dissolved in the desired concentration in water, along with the desired amount of electrolyte. The oxidation is then carried out at a voltage of about 2 to about 6 volts and at the maxi- Y mumicurrent density that can be used without increasing the potential to the point where undesirable by-products are formed. lt has been found that a current density of aboutV 0.02 amps/sq. cm. gives highly `satisfactory results. As a result of this process there is obtained a sulfoxideor sulfone of the starting organic sulfide in high yield in a single step process without any intermediate separation and purication requirements.

The invention is illustrated, but not limited, by the following examples in which the parts are expressed by Weight unless otherwise stated.

EXAMPLE I EXAMPLE Il In a glass electrolytic oxidation cell were suspended a platinum anode and a nickle cathode. The cell was charged with a mixture of 70% 2,2dihydroxyethyl conL sulde, 3% chloride and the balance water. The solution was anodically oxidized with direct current for 80 hours at ambient temperature, using 4 volts and 1 amp. As a result, dihydroxyethylsulfone was formed at a cell eliiciency of over 90%, based upon the sulfide consumed.

The oxidation of Example ll was repeated, using 3% solutions of each of the following electrolytes: sodium sulfate, ammonium sulfate, sulfuric acid, hydrochloric acid and arsenious acid. In Veach case dihydroxyethylsulfone was formed at a cell efliciency of at least about 90%, based upon the sulfide consumed.

On repeating Example Il utilizing the reaction product of either propylene oxide and hydrogen sulfide or propiolactone and hydrogen sullide, or epichlorohydrin and hydrogen'sulide, the corresponding sulfone is produced at high cell efficiency.

Care should be exercised in the choice of materials used for the electrolytic cell. Glassor plastic-coated metal has been found eminently suitable.

Although the above examples illustrate a batch operation, the invention is not thus limited. The process of the instant invention is eminently suited for continuous operation and Vit is contemplated that the oxidation cells can be designed and arranged for continuous operation with continuous or intermittent introduction of watersoluble organic suliide to one or a series of cells and continuous or intermittent Withdrawal of the ,sulfone-enriched electrolyte from the cell or from one of the series of cells. Under Such conditions, different operating conditions may be employed in different cells to obtain maximum current efficiency.

A process for the continuous production of-dihydroxyfrom a suitable supply, are then fed to a second reactor where -theyunite and form dihydroxyethylsuilide. (By substituting propylene oxide,butylene oxide epichlorohydrin, lactones, etc., for the ethylene oxide, other suitable organic suliides can be prepared, as known to those skilled in the art, and substituted for dihydroxyethylsulde herein.) The resulting organic sulfide, together with an aqueous solution of a suitable electrolyte, in'this case 3% sodium chloride solution (based on Ythe total weight ofthe solution), are then charged to the electrolytic oxidation reactor.

This reactor constitutes a number of cells connected in series. 'The cells may be either bipolar or unipolar and a standard commercial design'. Desirably, the last cell or cells in the series (i.e., the cells at the output end of the oxidation reactor) have substantially increased electrode areas as compared to the first cells in the series (i.e., the Celis-at the input end of the oxidation reactor). By increasing the electrode area, the smaller concentration of remaining sulide can more freely-contact the electrodes and there is less interference with the Vflow of current so that, byY appropriate design, the current efliciency can be kept high throughout the reaction. Without this design it has been found that in the case of the oxidation of a 70% dihydroxyethylsulfidesolution, current eiciency stays high until about %V conversion. The inal 10% conversion is attended by a marked dropofl` in current Veiliciency and a consequent Vlengthening of time for the oxidation to be completed. By modifying the electrode area in the manner described, the current elciency may be maintained high and the time for completion of the oxidation markedly reduced.

As 'the instant process is remarkably eicie'nt-in producing the sulfone at over 90% eiiiciency, the product from the last cell contains as impurities only the sodium chloride, water and no more than 10% impurities from the oxidation. This high quality product needs no further separation treatment and may be charged directly to storage as shown where it is suitable for immediateV commercial use. If a more highly chemically purified product is desired, further separation as by distillation, ion exchange, etc., .may` be used as known to those skilled in the art.

The course of the oxidation wherein water isV electrolyzed to provide Vthe oxygen toY convert the sulfide of the sulfone, produces considerable quantities of by-product hydrogen. This is taken from the electrolytic oxidation cells and recycled to the hydrogen input of the hydrogen sulfide reactor. Provision is also made in the process fork the isolation of the various intermediates (i.e., mercaptoethanol and,bis-hydroxyethyl'sulfoxide, for the particular reactants shown) at the corresponding Vstages of the synthesis where it is desired to Vobtain such materials as commercial products. Theresult is a continuous process for producing an organic sulfone in high yield at extremely high efficiency. i

it is contemplated that the addition of small proportions, i.e., less than about 1%.by weight, of oxidation promoters to the electrolyte may be beneficial. These and similar variations in the process will, of course, be obvious to those skilledin'the art.

I claim: Y j 1. A continuous process comprising passing hydrogen and sulfur into a first reactorVv to form hydrogen suliide,

passing the resulting hydrogen sulde and a 1,2-alkylene oxide to a second reactor to' form a 'dihydroxyalkylsulfide, then passing'theresulting dihydroxyalkylsulde and an aqueous lsolution of an electrolyte through an electrolytic oxidation reactor wherein the dihydroxaikylsulfide is Vsubjected to electrolysis whereby there is obtainedhydrogen and dihydroxyalkylsulfoxide', recirculating the hydrogen Vto the hydrogen input of the 'Virst reactor and removing the sulfoxide solution from the oxidation reactor. Y

2V. A process for the productionof :inorganic suifone which comprises subjecting ,an aqueousV solution Vof aV water-soluble organic thioether containing an electrolyte in an amount between about 0.1% by weight of the solution and the saturation point of the electrolyte in the solution, to direct current electrolysis at a temperature between about 5 C. and about 150 C. and a voltage between about 2 and 6 volts thereby converting said aqueous solution of a water-soluble organic thioether to an aqueous solution of the corresponding water-soluble organic sulfone.

3. A process according to claim 2 wherein the watersoluble organic thioether is dihydroxyethylsulde.

4. A process according to claim 2 wherein the watersoluble organic thioether is the reaction product of proplyene oxide and hydrogen sulfide.

5. A process according to claim 2 wherein the watersoluble organic thioether is the reaction product of hydrogen sulfide with epichlorohydrin.

6. A process according to claim 2 wherein the watersoluble organic thioether is thereaction product of hydrogen sulfide with -propiolactone.

7. A process according to claim 2 wherein the watersoluble organic thioether is thiodiacetic acid.

8. A process according to claim 2 wherein the watersoluble organic thioether is ethylthioethanol.

9. A continuous process for the production of an organic sulfone comprising passing hydrogen and sulfur into a lirst reactor to form hydrogen sulfide, passing the resulting hydrogen sulfide and a LZ-alkylene oxide to a second reactor to form a dihydroxyalkylsulfde, then passing the resulting dihydroxyalkylsulfide and an aqueous solution of an electrolyte through an electrolytic oxidation reactor wherein the dihydroxyalkylsulfide is subjected to electrolysis whereby there is obtained hydrogen and dihydroxyalkylsulfone, recirculating the hydrogen to the hydrogen input of the first reactor and removing the sulfone solution from the oxidation reactor.

10. The process according to claim 9 wherein the oxidation reactor has a plurality of cells, the cells at the output end of the reactor having substantially increased electrode areas relative to the electrode areas of the cells at the input end of the reactor.

11. The process to claim 9 wherein the alkylene oxide is selected from the group consisting of ethylene oxide, propylene oxide and epichlorohydrin.

l2. The process according to claim 9 wherein the alirylene oxide is ethylene oxide.

13. The process according to claim 9 wherein the temperature reactor is between 25 C. and 90 C. during the electrolysis.

14. The process according to claim 9 including means to separate the adduct of hydrogen sulfide and one mol of the 1,2-alkylene oxide from the second reactor.

l5. A continuous process for the production of an organic sulfoxide and organic sulone comprising passing hydrogen and sulfur into a first reactor to form hydrogen sulfide, passing the resulting hydrogen sulfide and a 1,2-alkylene oxide to a second reactor to forrn a dihydroxyalkylsulfide, then passing the resulting dihydroxyalkylsulfide and an aqueous solution of an electrolyte through an electrolytic oxidation reactor wherein the dihydroxyalkylsulfide is subject to electrolysis whereby there is obtained hydrogen, dihydroxyalkylsulfoxide and dihydroxyalkylsulfone, recirculating the hydrogen to the hydrogen input of the first reactor, removing the sulfoxide solution from a first point in the oxidation reactor and then at a subsequent point in said oxidation reactor removing the sulfone solution therefrom.

References Cited by the Examiner UNITED STATES PATENTS 9/50 Brown 204-79 OTHER REFERENCES WINSTON A. DOUGLAS, Primary Examiner.

JOHN R. SPECK, Examiner. 

2. A PROCESS FOR THE PRODUCTION OF AN ORGANIC SULFONE WHICH COMPRISES SUBJECTING AN AQUEOUS SOLUTION OF A WATER-SOLUBLE ORGANIC THIOETHER CONTAINING AN ELECTROLYTE IN AN AMOUNT BETWEEN ABOUT 0.1% BY LWEIGHT OF THE SOLUTION AND THE SATURATION POINT OF THE ELECTROLYTE IN THE SOLUTION, TO DIRECT CURRENT ELECTROLYSIS AT A TEMPERATURE BETWEEN ABOUT -5*C. AND ABOUT 150*C. AND A VOLTAGE BETWEEN ABOUT 2 AND 6 VOLTS THEREBY CONVERTING SAID AQUEOUS SOLUTION OF A WATER-SOLUBLE ORGANIC THIOETHER TO AN AQUEOUS SOLUTION OF THE CORRESPONDING WATER-SOLUBLE ORGANIC SULFONE. 